[postgis-devel] Two geometries intersects but intersection is empty

[Paul Ramsey]

Ah, yes, PostGIS is using its own code for the intersects test on
point/polygon... hence the incompatibility. Hm hm.

P

On Thu, Jun 3, 2010 at 3:15 PM, Martin Davis <mbdavis at refractions.net> wrote:
> Thanks.
>
> I was actually a bit puzzled by this, since in JTS the intersects and
> intersection operations use exactly the same code to test whether the point
> lies inside the polygon.  And in fact, when I run this case in JTS, it
> reports that:
>
> intersects = FALSE
> intersection = EMPTY
>
> which is consistent.
>
> It seems odd that PostGIS behaves differently.  It would be interesting to
> try this directly in GEOS.  Although does PostGIS perhaps not use GEOS for
> this particular case?
> Also, the code run in JTS is of fairly recent vintage, so it's possible that
> GEOS isn't reflecting the most up-to-date algorithm.
>
> Martin
>
> Pierre Racine wrote:
>>
>> SELECT st_intersects(geomA, geomB), astext(st_intersection(geomA, geomB))
>> FROM (SELECT
>>      st_geomfromtext('POINT(-75.5516150170498 49.2824791927083)',4326) AS
>> geomA,
>>
>>  geomfromewkb(decode('0103000020e610000001000000050000001c9f33ce69e352c0718f0e9a27a44840e77546654fe352c067d2ad7b35a448406cd4767448e352c0fc7fd3a900a44840a1fd63dd62e352c0073d34c8f2a348401c9f33ce69e352c0718f0e9a27a44840',
>> 'hex')) AS geomB
>> ) foo
>>
>>
>>
>>>
>>> -----Original Message-----
>>> From: postgis-devel-bounces at postgis.refractions.net
>>> [mailto:postgis-devel-
>>> bounces at postgis.refractions.net] On Behalf Of Paul Ramsey
>>> Sent: 3 juin 2010 14:26
>>> To: PostGIS Development Discussion
>>> Subject: Re: [postgis-devel] Two geometries intersects but intersection
>>> is empty
>>>
>>> FYI: select encode(asbinary(geom),'hex') from ...
>>>
>>> P
>>>
>>> On Thu, Jun 3, 2010 at 2:24 PM, Martin Davis <mbdavis at refractions.net>
>>> wrote:
>>>
>>>>
>>>> Pierre, can you provide the second geometry in WKBHex?  I can't read the
>>>> string that you provided.
>>>>
>>>> Pierre Racine wrote:
>>>>
>>>>>
>>>>> Here is the case:
>>>>>
>>>>> SELECT st_intersects(geomA, geomB), astext(st_intersection(geomA,
>>>>> geomB))
>>>>> FROM (SELECT
>>>>>     st_geomfromtext('POINT(-75.5516150170498 49.2824791927083)',4326)
>>>>> AS
>>>>> geomA,
>>>>>     geomfromewkb(E'\\001\\003\\000\\000
>>>>>
>>>>>
>>>
>>>
>>> \\346\\020\\000\\000\\001\\000\\000\\000\\005\\000\\000\\000\\034\\2373\\316i\\343R\\300q\\217\\016\\2
>>>
>>> 32\'\\244H@\\347uFeO\\343R\\300g\\322\\255{5\\244H at l\\324vtH\\343R\\300\\374\\177\\323\\251\\000\\244H
>>>
>>> @\\241\\375c\\335b\\343R\\300\\007=4\\310\\362\\243H@\\034\\2373\\316i\\343R\\300q\\217\\016\\232\'\\2
>>> 44H@')
>>>
>>>>>
>>>>> AS geomB
>>>>> ) foo
>>>>>
>>>>> Sorry I can't provide it as wkt as the wkt version return the expected
>>>>> result (and the wkb version does not (before I read Paul's last FAQ)).
>>>>>
>>>>> Pierre
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> -----Original Message-----
>>>>>> From: postgis-devel-bounces at postgis.refractions.net
>>>>>> [mailto:postgis-devel-
>>>>>> bounces at postgis.refractions.net] On Behalf Of Martin Davis
>>>>>> Sent: 3 juin 2010 12:15
>>>>>> To: PostGIS Development Discussion
>>>>>> Subject: Re: [postgis-devel] Two geometries intersects but
>>>>>> intersection
>>>>>> is empty
>>>>>>
>>>>>> Can you post the case?
>>>>>>
>>>>>> This can happen because the intersects predicate test is more accurate
>>>>>> than the intersection computation.  (Technically speaking, it's
>>>>>> possible
>>>>>> to detect that two segments intersect, but be unable to compute an
>>>>>> accurate point that represents the intersection).
>>>>>>
>>>>>> Pierre Racine wrote:
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> Is it possible that st_intersects returns true on two geometries but
>>>>>>> their st_intersection returns
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> "GEOMETRYCOLLECTION EMPTY"?
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> I have a nice case here...
>>>>>>>
>>>>>>> Pierre
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> postgis-devel mailing list
>>>>>>> postgis-devel at postgis.refractions.net
>>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> --
>>>>>> Martin Davis
>>>>>> Senior Technical Architect
>>>>>> Refractions Research, Inc.
>>>>>> (250) 383-3022
>>>>>>
>>>>>> _______________________________________________
>>>>>> postgis-devel mailing list
>>>>>> postgis-devel at postgis.refractions.net
>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>>>>
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> postgis-devel mailing list
>>>>> postgis-devel at postgis.refractions.net
>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>>>
>>>>>
>>>>>
>>>>
>>>> --
>>>> Martin Davis
>>>> Senior Technical Architect
>>>> Refractions Research, Inc.
>>>> (250) 383-3022
>>>>
>>>> _______________________________________________
>>>> postgis-devel mailing list
>>>> postgis-devel at postgis.refractions.net
>>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>>
>>>>
>>>
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>>
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>>
>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>